In electrophotographic printing machines which have been widely put into practice previously, several efforts have been made in order to simplify their image-forming processes.
For example, as shown in FIG. 7, Japanese Laid-Open Patent Publication, No. 302274/1989 (Tokukaihei 1-302274), has disclosed a method wherein a latent-image forming body 50 having a ferro-electric layer 51 on its surface and a thermal head 52 are used and by utilizing the fact that the dielectric constant of the ferro-electric substance varies with temperature, an electrostatic latent image is formed. In other words, in this electrostatic-latent-image forming method, information is written to the ferro-dielectric layer 51 by making the thermal head 52 cause temperature changes locally on the ferro-electric layer 51. Immediately after the information has been written thereto, the written portion is charged by a corona charger 53. At this time, since the ferro-electric layer 51 has its dielectric constant changed depending on temperatures, the electric potential of the drum surface of the latent-image forming body 50, caused by the charge, becomes smaller at heated portions and also becomes relatively greater at the other portions. Thus, an electrostatic latent image is formed by utilizing these temperature differences. Here, this phenomenon which exerts electric potentials on the surface in response to temperature changes is referred to as the pyroelectric effect. With this method, the service life of the latent-image forming body 50 is lengthened by eliminating the necessity of the exposing means.
Meanwhile, as shown in FIG. 8 and FIG. 9, Japanese Examined Patent Publications, No. 4900/1990 (Tokukouhei 2-4900) and No. 64864/1991 (Tokukouhei 3-64864), have disclosed a method wherein conductive carriers e are generated in a photoconductive layer 62 or 72 by irradiating light L from the back of a transparent electrode 61 or 71 and an image is formed by attracting conductive toner 63 or 73 that has been subjected to injection of charge. The conductive toner 63 or 73, used in this image formation, exhibits magnetism, and is carried toward attracting portions through a magnetic force exerted by a magnet 64, a magnet roller 74, or other members.
However, in the former method of the above-mentioned conventional image-forming methods wherein the changes in dielectric constant in the ferro-electric substance, caused by temperature changes, are utilized, it is necessary to install an expensive thermal head 52 and a charging device, such as a corona charger 53, for use with the latent-image forming body 50. Therefore, the problem of this device is that the cost reduction is difficult.
Further, in the latter method, it is necessary to allow the conductive toner 63 or 73 serving as developer to exhibit conductivity and magnetism. This increases the cost of the developer, resulting in high cost in the image-forming process.